Iron-copper-based oil-impregnated sintered bearing and method for manufacturing same

ABSTRACT

A sintered bearing exhibits less of a hard iron alloy phase, and has an excellent wear resistance and cost performance under low-revolution and high-load use conditions; and a method for producing such a sintered bearing. The sintered bearing contains Cu: 10 to 55% by mass, Sn: 0.5 to 7% by mass, Zn: 0 to 4% by mass, P: 0 to 0.6% by mass, C: 0.5 to 4.5% by mass and a remainder composed of Fe and inevitable impurities. An area ratio of a free graphite dispersed in a metal matrix of the bearing is 5 to 35%; a porosity thereof is 16 to 25%; a hardness of an iron alloy phase in the matrix is Hv 65 to 200; and raw material powders employ at least one of a crystalline graphite powder and a flake graphite powder each having an average particle size of 10 to 100 μm.

CROSS-REFERENCE TO RELATED PATENT APPLICATIONS

This application is a U.S. National Phase Application under 35 U.S.C. §371 of International Patent Application No. PCT/JP2017/027339, filedJul. 27, 2017, and claims the benefit of Japanese Patent ApplicationsNo. 2016-150516, filed Jul. 29, 2016 and No. 2017-017106, filed Feb. 1,2017, all of which are incorporated by reference herein in theirentireties. The International Application was published in Japanese onFeb. 1, 2018 as International Publication No. WO/2018/021501 under PCTArticle 21(2).

FIELD OF THE INVENTION

The present invention relates to an iron-copper-based oil-impregnatedsintered bearing used under a low-revolution and high-load use conditionsuch as that involving an output shaft of an electric motor of anautomobile; and a method for producing this sintered bearing.

BACKGROUND OF THE INVENTION

In recent years, iron-copper-based oil-impregnated sintered bearingshave been used also on output shafts of electric motors for automobiles.As such iron-copper-based oil-impregnated sintered bearing, thosedisclosed in, for example, Patent documents 1 and 3 are known. In thecase of an automobile such as a minivan, power windows and wiper motorsinvolving larger window glasses are operated with outputs higher thanbefore, thus resulting in a higher load and surface pressure applied toa bearing of an output shaft. Further, there has been a problem thatsince the output shafts used in these motors are revolved at a low speedof 200 rpm or lower, an oil film unique to an oil-impregnated sinteredbearing tends to be formed in an insufficient manner such that the wearresistance of the oil-impregnated sintered bearing is now at risk as theshaft and bearing slide against each other in the form of metal contact.In order to impart a lubricity to the iron-copper-based oil-impregnatedsintered bearing under such sliding condition with a lesser oillubrication effect, there has been known a technique of dispersing anddistributing graphite in a bearing material. An iron-copper-basedsintered bearing containing graphite is produced by press-molding a rawmaterial powder(s) that has been mixed with a graphite powder; and thenperforming steps such as sintering, sizing and oil impregnation on amolded body obtained. However, since the material composition of thebearing is that of an iron-copper-based sintered alloy, an iron powderand a graphite powder react with each other during sintering such that ahard iron alloy phase can be formed easily. An iron-copper-basedoil-impregnated sintered bearing having a hard alloy phase therein oftendamages a sliding partner shaft member and then undergoes wear itselfdue to the damaged shaft, under a sliding condition such as thatinvolving an output shaft of an electric motor for an automobile. As aremedial measure, although a quenched steel member with a high hardnessmay be used as a shaft, a carbon steel shaft has been used for costreduction.

Here, as a conventional technique for inhibiting a reaction between aniron powder and a graphite during sintering, there has been known atechnique where a raw material powder is at first prepared by mixing aniron powder, 2.0 to 9.0% by mass of a flake copper powder with anaverage particle size of 20 to 150 μm and 1.5 to 3.7% by mass of agraphite powder with an average particle size of 40 to 80 μm; and thenby performing sintering at a temperature of 950 to 1030° C., there willbe formed in the bearing an iron alloy phase having ferrite at an arearatio of 20 to 85% and a remainder composed of pearlite (Patent document2). However, since pearlite is a hard phase formed by the reaction ofiron and graphite, a sliding partner shaft member cannot be sufficientlyprevented from being damaged.

PRIOR ART DOCUMENT Patent Document

Patent document 1: JP-A-2003-221606

Patent document 2: JP-A-2010-77474

Patent document 3: JP-A-2013-92163

Patent document 4: WO1999/008012

SUMMARY OF THE INVENTION Problem to be Solved by the Invention

It is an object of the present invention to provide a noveliron-copper-based oil-impregnated sintered bearing exhibiting a less ofa hard iron alloy phase, and having an excellent wear resistance andcost performance under a low-revolution and high-load use condition suchas that involving an output shaft used in an electric motor of anautomobile; and a method for producing such sintered bearing.

Means to Solve the Problem

The iron-copper-based oil-impregnated sintered bearing of the presentinvention contains:

Cu: 10 to 55% by mass;

Sn: 0.5 to 7% by mass;

Zn: 0 to 4% by mass;

P: 0 to 0.6% by mass;

C: 0.5 to 4.5% by mass; and

a remainder composed of Fe and inevitable impurities, wherein an arearatio of a free graphite dispersed in a metal matrix is 5 to 35%.

Further, a porosity is 16 to 25%.

Further, a hardness of an iron alloy phase in the matrix is Hv 65 to200.

The method for producing the iron-copper-based oil-impregnated sinteredbearing of the present invention, includes:

a step of obtaining a mixed powder by mixing raw material powders suchthat the mixed powder contains Cu: 10 to 55% by mass, Sn: 0.5 to 7% bymass, Zn: 0 to 4% by mass, P: 0 to 0.6% by mass, C: 0.5 to 4.5% by massand a remainder composed of Fe and inevitable impurities;

a step of obtaining a green compact by press-molding the mixed powder;and

a step of obtaining a sintered body by sintering the green compact at atemperature within a range of 820 to 940° C., wherein the raw materialpowders employ at least one of a crystalline graphite powder and a flakegraphite powder each having an average particle size of 10 to 100 μm,and an area ratio of a free graphite dispersed in a metal matrix of theiron-copper-based oil-impregnated sintered bearing is 5 to 35%.

Effects of the Invention

The iron-copper-based oil-impregnated sintered bearing of the presentinvention contains Cu: 10 to 55% by mass; Sn: 0.5 to 7% by mass; Zn: 0to 4% by mass; P: 0 to 0.6% by mass; C: 0.5 to 4.5% by mass; and aremainder composed of Fe and inevitable impurities, in which an arearatio of a free graphite dispersed in a metal matrix is 5 to 35%.Therefore, the hardness of the iron alloy phase can be controlled in anappropriate range, and the bearing itself thus has an excellent wearresistance and cost performance under a low-revolution and high-load usecondition such as that involving an output shaft used in an electricmotor of an automobile.

The method for producing the iron-copper-based oil-impregnated sinteredbearing includes a step of obtaining a mixed powder prepared by mixingraw material powders such that the mixed powder contains Cu: 10 to 55%by mass, Sn: 0.5 to 7% by mass, Zn: 0 to 4% by mass, P: 0 to 0.6% bymass, C: 0.5 to 4.5% by mass and a remainder composed of Fe andinevitable impurities, a step of obtaining a green compact bypress-molding the mixed powder, and a step of obtaining a sintered bodyby sintering the green compact at a temperature within a range of 820 to940° C., in which the raw material powders employ at least one of acrystalline graphite powder and a flake graphite powder each having anaverage particle size of 10 to 100 μm, and an area ratio of a freegraphite dispersed in a metal matrix of the iron-copper-basedoil-impregnated sintered bearing is 5 to 35%. Therefore, there can beproduced an iron-copper-based oil-impregnated sintered bearing having aniron alloy phase whose hardness can be controlled in an appropriaterange; and thus exhibiting an excellent wear resistance and costperformance under a low-revolution and high-load use condition such asthat involving an output shaft used in an electric motor of anautomobile.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are explanatory diagrams graphically illustrating acolor photograph of a bearing surface for the description of a methodfor measuring an area ratio of a free graphite, in which FIG. 1A shows ameasurement state, and FIG. 1B shows a state where squares have beenpainted individually.

FIG. 2 is a photograph of an inner diameter surface of aniron-copper-based oil-impregnated sintered bearing of a working examplein the present invention, the sintered bearing having a free graphitebeing dispersed and distributed therein.

DETAILED DESCRIPTION OF THE INVENTION

The iron-copper-based oil-impregnated sintered bearing of the presentinvention contains 10 to 55% by mass of Cu; 0.5 to 7% by mass of Sn; 0to 4% by mass of Zn; 0 to 0.6% by mass of P; and 0.5 to 4.5% by mass ofC; and a remainder composed of Fe and inevitable impurities, in which anarea ratio of a free graphite dispersed in a metal matrix is 5 to 35%.

Although the graphite may be a natural graphite such as a crystallinegraphite, a flake graphite and an amorphous graphite; or an artificialgraphite, the iron-copper-based oil-impregnated sintered bearing of thepresent invention employs at least one of a natural crystalline graphitepowder and flake graphite powder with a favorable crystallinity and anexcellent lubricity, due to the fact that the lubricity of a graphiteaffects the degree of crystal growth. Further, in order to inhibit asintering-induced reaction between iron and graphite in raw materialpowders, at least one of a crystalline graphite powder and a flakegraphite powder each having an average particle size of 10 to 100 μm isused in the raw material powders of an iron-copper-based sintered alloy,and a sintering temperature is set to 820 to 940° C., thereby making itpossible to secure a certain value of the area ratio of the freegraphite dispersed in the metal matrix of a sintered body, and alsocontrol the hardness of an iron alloy phase.

Here, the average particle size of the graphite powder refers to avolume mean diameter MV as a value measured by a laser diffractionmethod.

The composition of the iron-copper-based oil-impregnated sinteredbearing of the present invention is described in detail hereunder.

(1) Cu: 10 to 55% by Mass

Cu forms a solid solution together with Sn and P. Such Cu—Sn—P solidsolution is softer than a sliding partner shaft, thereby improving aconformability with the sliding partner shaft, and thus contributing toan improvement in wear resistance of the bearing. When the amount of Cucontained is smaller than 10% by mass, there cannot be achieved thedesired effects. Further, it is not preferable when the amount of Cucontained is larger than 55% by mass, because there will be exhibited aninsufficient bearing strength such that bearing wear under a high loadcondition will be more significant.

(2) Sn: 0.5 to 7% by Mass

Sn forms a matrix solid solution together with Cu and P, and improvesthe strength of the bearing, thereby contributing to the improvement inwear resistance of the bearing. When the amount of Sn contained issmaller than 0.5% by mass, there cannot be achieved the desired effects.Further, it is not preferable when the amount of Sn contained is largerthan 7% by mass, because there will be no effect in improving thestrength, and a dimension accuracy will actually be impaired.

(3) Zn: 0 to 4% by Mass

Zn forms a matrix solid solution together with Cu and Sn, and has aneffect of improving the corrosion resistance, conformability andstrength of the bearing. When the amount of Zn contained is larger than4% by mass, there will be no desired effect improvement. Thus, if Zn isadded, it is preferred that it be added in an amount of not larger than4% by mass.

(4) P: 0 to 0.6% by Mass

P is added in the form of a Cu—P alloy powder; and has an effect ofyielding a liquid phase during sintering so as to promote sintering, andan effect of forming a matrix solid solution together with Cu and Sn soas to improve wear resistance. It is not preferable when the amount of Pcontained is larger than 0.6% by mass, because there will be no desiredeffect improvement, and a higher degree of deformation will actually beobserved during sintering. Thus, if P is added, it is preferred that itbe added in an amount of not larger than 0.6% by mass.

(5) C: 0.5 to 4.5% by Mass

When dispersed as a free graphite in the matrix of the bearing alloy, Cimparts a superior lubricity to the bearing so as to contribute to theimprovement in wear resistance and a reduction in friction coefficient.When the amount of C contained is smaller than 0.5% by mass, therecannot be achieved the effect of improving wear resistance. Further, itis not preferable when the amount of C contained is larger than 4.5% bymass, because strength will decrease significantly.

(6) Porosity: 16 to 25%

Pores, when dispersed in the matrix, have an effect of absorbing astrong friction against the bearing so as to inhibit wear of thebearing. However, such effect will be insufficient if the porosity islower than 16%. Further, it is not preferable when the porosity ishigher than 25%, because strength will decrease significantly.

(7) Crystalline Graphite Powder and Flake Graphite Powder

As a graphite powder(s) used in the raw material powders, there can beemployed at least one of a crystalline graphite powder and a flakegraphite powder. The crystalline graphite powder and flake graphitepowder are to be dispersed and distributed as free graphite in thematrix of the bearing alloy, thereby imparting an excellent lubricity tothe bearing, and thus contributing to the improvement in wear resistanceand the reduction in friction coefficient. When the average particlesize of the crystalline graphite powder or flake graphite powder issmaller than 10 μm, the above effects of the graphite powder cannot beachieved due to the fact that the hardness of the iron alloy phase willincrease by a reaction between the graphite powder(s) and an iron powderin the raw material powders during sintering, and the fact that therewill now be a lower area ratio of free graphite. Further, it is notpreferable when such average particle size is larger than 100 μm,because strength will decrease significantly.

Described hereunder are specific working examples of theiron-copper-based oil-impregnated sintered bearing of the presentinvention and a method for producing the same. However, the invention isnot limited to the following working examples. In fact, the inventionmay be carried out in various modified ways.

WORKING EXAMPLES

As raw material powders, there were prepared a reduced iron powder andan electrolytic copper powder for powder metallurgy; a flake copperpowder; a Cu-9% by mass Sn powder; a Sn powder; a Cu-8% by mass Ppowder; a Cu-20% by mass Zn powder; and a graphite powder. As for agraphite powder among these powders, each of the working examples of thepresent invention used a crystalline graphite powder and/or flakegraphite powder with a larger average particle size, whereas each of thecomparative examples used an artificial graphite powder or crystallinegraphite powder with a smaller average particle size.

These raw material powders were combined together in a way such that thefinal component composition(s) shown in Table 1 was met. Next, 0.5% ofzinc stearate was added thereto, and a V-type mixer was then used to mixthe components for 20 min so as to obtain a mixed powder. The mixedpowder was then turned into a green compact by press molding, and thisgreen compact was then sintered at a given temperature within a range of820 to 940° C. under an endothermic gas atmosphere prepared by mixing anatural gas with air, and then passing the mixed gas through a heatedcatalyst so as to decompose and convert the gas. Thus, a sintered bodywas obtained. Sizing was then carried out at a given pressure, followedby impregnating the sized piece with a particular synthetichydrocarbon-based lubricant oil, thereby obtaining ring-shaped testpieces in working examples 1 to 17 and comparative examples 1 to 8.These ring-shaped test pieces were iron-copper-based oil-impregnatedsintered bearings each having a size of outer diameter: 18 mm×innerdiameter: 8 mm×height: 8 mm; and the component composition(s) shown inTable 1 with a free graphite dispersed therein.

TABLE 1 Composition of raw material powder (% by mass) Average Graphitediameter of Cu Cu flat Fe Cu—P Cu—Sn Sn Cu—Zn (C) graphite Bearingpowder powder powder powder powder powder powder powder Total powder(μm) Working 1 5 5 86.5 Cu—8% P: 0 Cu—9% Sn: 0 0.5 Cu—20% Zn: 0 3 100.020 example 2 5 10 83 Cu—8% P: 0 Cu—9% Sn: 0 1 Cu—20% Zn: 0 1 100.0 15 ofthe 3 10 10 77 Cu—8% P: 0 Cu—9% Sn: 0 1 Cu—20% Zn: 0 2 100.0 10invention 4 6.5 0 63.8 Cu—8% P: 2.5 Cu—9% Sn: 16.7 0 Cu—20% Zn: 7.5 3100.0 70 5 0.4 5 73.6 Cu—8% P: 5.0 Cu—9% Sn: 11 3 Cu—20% Zn: 0 2 100.085 6 17.4 0 54.1 Cu—8% P: 5.0 Cu—9% Sn: 11 1 Cu—20% Zn: 10 1.5 100.0 507 18.3 0 48.2 Cu—8% P: 3.75 Cu—9% Sn: 16.7 1 Cu—20% Zn: 10 2 100.0 70 810.1 0 42.1 Cu—8% P: 5.0 Cu—9% Sn: 27.8 0 Cu—20% Zn: 12.5 2.5 100.0 85 97.8 10 36.4 Cu—8% P: 7.5 Cu—9% Sn: 33.3 3 Cu—20% Zn: 0 2 100.0 50 10 6.315 46.7 Cu—8% P: 3.75 Cu—9% Sn: 22.2 2 Cu—20% Zn: 0 4 100.0 85 11 6.2 1048.6 Cu—8% P: 5.0 Cu—9% Sn: 22.2 1 Cu—20% Zn: 5 2 100.0 50 12 0 15.447.2 Cu—8% P: 3.75 Cu—9% Sn: 11.1 0 Cu—20% Zn: 20 2.5 100.0 25 13 13.3 046.2 Cu—8% P: 3.75 Cu—9% Sn: 22.2 0 Cu—20% Zn: 10 4.5 100.0 95 14 0 1749.0 Cu—8% P: 6.25 Cu—9% Sn: 20.0 5.2 Cu—20% Zn: 0 2.5 100.0 85 15 12.520 46.5 Cu—8% P: 5.0 Cu—9% Sn: 0 3.5 Cu—20% Zn: 10 2.5 100.0 70 16 17 1048.0 Cu—8% P: 5.0 Cu—9% Sn: 10 4 Cu—20% Zn: 5 1 100.0 70 17 12.3 10 48.6Cu—8% P: 5.0 Cu—9% Sn: 11.1 1 Cu—20% Zn: 10 2 100.0 85 Comparative 1 7 089.5 Cu—8% P: 0 Cu—9% Sn: 0 0.5 Cu—20% Zn: 0 3 100.0 50 example 2 39.3 531.6 Cu—8% P: 5.0 Cu—9% Sn: 11.1 1 Cu—20% Zn: 5 2 100.0 5 3 18.5 5 49.2Cu—8% P: 3.75 Cu—9% Sn: 0 0 Cu—20% Zn: 20 3.5 100.0 10 4 11.2 0 44.7Cu—8% P: 3.75 Cu—9% Sn: 33.3 5 Cu—20% Zn: 0 2 100.0 5 5 19.5 0 41.7Cu—8% P: 3.75 Cu—9% Sn: 0 3 Cu—20% Zn: 30 2 100.0 10 6 2.7 0 47 Cu—8% P:0 Cu—9% Sn: 33.3 0 Cu—20% Zn: 15 2 100.0 5 7 4.3 0 53.7 Cu—8% P: 3.75Cu—9% Sn: 22.2 1 Cu—20% Zn: 15 0 100.0 — 8 33.7 0 39.8 Cu—8% P: 2.5Cu—9% Sn: 0 3 Cu—20% Zn: 15 6 100.0 120

Wear test was performed on the ring-shaped oil-impregnated sinteredbearing thus obtained, under a low-speed and high-load condition.

A S45C carbon steel shaft having an outer diameter of 8 mm was insertedinto the ring-shaped oil-impregnated sintered bearing, followed byrotating the shaft at a rate of 5 m/min for 50 hours while applying aload of 100 kgf to the ring-shaped bearing from outside. Later, wearresistance evaluation was performed by measuring a maximum wear depth inthe sliding receiving surface of the ring-shaped bearing; and a maximumwear depth in the sliding portion of the sliding partner shaft.

Table 2 shows the component composition of each sample; porosities; arearatios of the free graphite; hardness of the iron alloy phase in thematrix; and maximum wear depths in the bearing and sliding partnershafts that were observed after the wear test. Here, the hardness of theiron alloy phase in the matrix was measured as follows. That is, anabrasive operation was performed on an end face of the bearing to anextent that a metallic structure would eventually become visible. AMicro Vickers tester was then used to select and target the iron alloyphase, and then measure the hardness thereof at three points under ameasuring load of 50 g. An average value was then obtained as thehardness of the iron alloy phase. The area ratio of the free graphitewas calculated as follows.

A CCD camera was used to obtain a color photograph of the inner diametersurface of the bearing (at 100-fold magnification), followed by placingthe frame of a given 2 mm grid tracing paper on top of the photograph,and then calculating the area ratio of the graphite parts. An examplethereof is described with reference to FIG. 1. FIG. 1(A) is a diagramrepresenting the color photograph of the surface of the bearing.Observed on such surface were a copper area 11 composed of copper or ancopper alloy; an iron area 12 composed of iron or an iron alloy; agraphite area 13 composed of graphite; and a pore area 14 composed ofpores. Squares 22 aligned in the longitudinal and transverse directionsare formed in a given area of a frame 21 made of, for example, atransparent panel. In FIG. 1, the squares 22 are, for example, providedin a manner of: longitudinal 10×transverse 10. In each square 22, a typeof area occupying the largest part thereof is counted as suchcorresponding area, and then calculated was the area ratio of thegraphite area 13 on the surface excluding the pore area 14. For the sakeof explanation, FIG. 1(B) is a diagram where the squares 22 areindividually painted in accordance with the type of each area 11, 12, 13and 14. For example, in FIG. 1(B), the numbers of the squares 22 countedas the copper area 11, iron area 12, graphite area 13 and pore area 14are respectively 40, 30, 10 and 20. Here, since the area ratio ofgraphite refers to a ratio of areas occupied by the graphite area 13excluding the pore area 14, the area ratio of graphite is10/80×100=12.5%. Three parts on the inner diameter surface of thebearing were photographed, followed by calculating an average graphitearea ratio via the method mentioned above.

As an example, FIG. 2 is a photograph of the inner diameter surface ofan iron-copper-based oil-impregnated sintered bearing of a workingexample of the invention, with free graphite being dispersed anddistributed therein.

TABLE 2 Radial Hardness Maximum wear Component Sintering Area ratiocrushing of iron depth (μm) composition (% by mass) temperature of freePorosity strength phase Counter Bearing Cu Sn Zn P C Fe (° C.) raphite(%) (%) (MPa) (Hv0.05) Bearing shaft Working 1 10 0.5 0 0 3 Remainder920 12.6 16 322 157 5 2 example 2 15 1 0 0 1 Remainder 900 5 19 376 1251 1 of the 3 20 1 0 0 2 Remainder 900 9.8 21 316 132 7 2 invention 4 301.5 1.5 0.2 3.0 Remainder 900 14.6 25 293 159 3 2 5 20 4.0 0.0 0.4 2.1Remainder 840 11.5 20 375 172 2 2 6 40 2.0 2.0 0.4 1.5 Remainder 940 7.719 396 196 6 3 7 45 2.5 2.0 0.3 2.0 Remainder 900 12.7 21 369 157 3 2 850 2.5 2.5 0.4 2.5 Remainder 900 16.3 20 344 143 2 1 9 55 6.4 0.0 0.61.9 Remainder 880 11.4 18 360 109 4 2 10 45 4.0 0.0 0.3 4.0 Remainder900 18.1 16 273 117 3 1 11 45 3.2 1.0 0.4 2.1 Remainder 915 10.8 21 366135 3 1 12 45 1.0 4.0 0.3 2.5 Remainder 870 18.1 19 310 120 2.5 0.3 1345 2.0 2.0 0.3 4.5 Remainder 880 30.2 23 291 93 5 1 14 41 6.9 0.0 0.52.4 Remainder 900 15.8 20 378 126 4 1 15 45 3.5 2.0 0.4 2.5 Remainder880 17.2 19 244 65 7 0.5 16 45 5.1 1.0 0.4 0.9 Remainder 880 7.0 18 380130 10 2 17 45 2.0 2.0 0.4 2.5 Remainder 820 12.7 20 284 88 8 1Comparative 1 7 0.5 0.0 0.0 3.0 Remainder 980 2.1 29 305 331 377 32example 2 63 2.9 3.0 0.4 2.2 Remainder 820 5.8 15 190 91 567 9 3 43 0.04.0 0.3 3.5 Remainder 920 10.3 20 217 246 36 12 4 45 8.1 0.0 0.3 1.9Remainder 940 5.8 22 380 280 246 15 5 47 3.0 5.8 0.3 2.0 Remainder 9306.7 21 419 277 482 24 6 45 2.9 3.0 0.0 2.0 Remainder 920 6.3 25 255 239558 11 7 40 3.0 2.8 0.3 0.0 Remainder 860 0 18 374 58 526 51 8 48 3.13.0 0.2 6.0 Remainder 940 36.8 22 114 216 461 3

As is clear from the test results regarding the maximum wear depths ofthe bearings and the sliding partner shafts, the bearings of the workingexamples of the present invention exhibited a superior wear resistance,and their sliding partner shafts exhibited less of wear, under thelow-speed and high-load test condition, as compared to the bearings ofthe comparative examples.

1. An iron-copper-based oil-impregnated sintered bearing comprising: Cu:10 to 55% by mass; Sn: 0.5 to 7% by mass; Zn: 0 to 4% by mass; P: 0 to0.6% by mass; C: 0.5 to 4.5% by mass; and a remainder composed of Fe andinevitable impurities, wherein an area ratio of a free graphitedispersed in a metal matrix is 5 to 35%.
 2. The iron-copper-basedoil-impregnated sintered bearing according to claim 1, wherein aporosity of the sintered bearing is 16 to 25%.
 3. The iron-copper-basedoil-impregnated sintered bearing according to claim 1, wherein ahardness of an iron alloy phase in the matrix is Hv 65 to
 200. 4. Amethod for producing an iron-copper-based oil-impregnated sinteredbearing, comprising: a step of obtaining a mixed powder by mixing rawmaterial powders such that the mixed powder contains Cu: 10 to 55% bymass, Sn: 0.5 to 7% by mass, Zn: 0 to 4% by mass, P: 0 to 0.6% by mass,C: 0.5 to 4.5% by mass and a remainder composed of Fe and inevitableimpurities; a step of obtaining a green compact by press-molding themixed powder; and a step of obtaining a sintered body by sintering thegreen compact at a temperature within a range of 820 to 940° C., whereinthe raw material powders employ at least one of a crystalline graphitepowder and a flake graphite powder each having an average particle sizeof 10 to 100 μm, and an area ratio of a free graphite dispersed in ametal matrix of the iron-copper-based oil-impregnated sintered bearingis 5 to 35%.
 5. The iron-copper-based oil-impregnated sintered bearingaccording to claim 2, wherein a hardness of an iron alloy phase in thematrix is Hv 65 to 200.